Accept GL_FRAGMENT_SHADER_DERIVATIVE_HINT in ES3 contexts.
Accept GL_FRAGMENT_SHADER_DERIVATIVE_HINT in ES3 contexts.

BUG=angleproject:2070

TEST=deqp/functional/gles3/shaderderivate_dfdx
TEST=deqp/functional/gles3/shaderderivate_dfdy
TEST=deqp/functional/gles3/shaderderivate_fwidth

Change-Id: I4e10343036a813c122ca41913324051b5c02e785
Reviewed-on: https://chromium-review.googlesource.com/538861
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

MSOpenTech/angle,ecoal95/angle,ecoal95/angle,ppy/angle,ecoal95/angle,ppy/angle,ecoal95/angle,MSOpenTech/angle,ppy/angle,MSOpenTech/angle,ppy/angle,MSOpenTech/angle,ecoal95/angle